Allow adding connections from config files (again)

[mathemancer]

Problem

As part of the architectural overhaul, we reworked the Connection model, splitting it into a number of components. However, this broke the script that automatically installed Mathesar schemas on databases defined in configuration files or environment variables.

Proposed solution

We should implement a solution to allow users to define database connections via some configuration file, and install Mathesar on the connected DB automatically. We should further consider how we want such DBs to be handled in the Web UI.